Each radio communication terminal disposed in a radio LAN system senses the carrier of a radio channel in advance of transmission of radio packets via the radio channel, and, when the radio channel is being used (channel busy), refrains from the transmission of the radio packets, whereas when the radio channel is being not used (channel idle), it transmits the radio packets.
Such a method of transmitting radio packets is generally called CSMA (Carrier Sense Multiple Access), and, for example, in accordance with the U.S. radio LAN standard IEEE802.11, CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is used (for example, refer to nonpatent references 1 and 2).
On the other hand, in accordance with the European radio LAN standard HiperLAN, a TDMA method which provides a transmission opportunity in response to a transmission request from a radio communication terminal is used.
In accordance with the U.S. radio LAN standard IEEE802.11, a communications method of using two or more channels including an extended channel (“Extension Channel” or “Secondary”) in addition to a fundamental channel (“Control Channel” or “Primary”) has been proposed in order to achieve further-high-speed transmission of radio packets.
Furthermore, in accordance with the U.S. radio LAN standard IEEE802.11, a communications method using a plurality of channels has been also proposed because the necessity of efficient transmission according to the channel capacity becomes high with utilization of an MIMO (Multiple Input, Multiple Output) technology and increase in the channel capacity because of use of a plurality of fundamental channels.
Concretely, the following communications method has been proposed.
For example, a method of enabling an existing radio communication terminal L-STA (Legacy STA (AP)) which carries out radio communications using a channel of a frequency band of 20 Hz, a radio communication terminal HT-STA (High Throughput STA (AP)) which carries out high-speed transmission, such as MIMO, (the frequency band of a channel which is used for the radio communications is 20 Hz), and a radio communication terminal HT-MC-STA (High Throughput Multi channel STA (AP)) which uses a channel of a frequency band of 40 Hz and carries out high-speed radio communications to coexist has been examined. Although the description about STA (AP) shows that each terminal can be an STA (Station) which is a radio LAN terminal, or can be an AP (Access point) which is an access point, a terminal which runs only in either STA or AP depending on the access method is also included.
FIG. 3 shows a frame format of the radio communication terminal L-STA, FIG. 4 shows a frame format of the radio communication terminal HT-STA, and FIG. 5 shows a frame format of the radio communication terminal HT-MC-STA.
In FIGS. 3 to 5, an L-STF (Legacy-Short Training Field) and an L-LTF (Legacy-Long Training Field) which are common fields are proposed by an existing standard, and are fields which all the radio communication terminals can recognize. The L-STF and the L-LTF are also fields which are used for phase synchronization, time synchronization, etc. with data which will be transmitted from now on.
An L-SIG (Legacy SIGNAL field) which is a common field is a field in which a modulation method (Rate), a data length (Length), etc. about the data which will be transmitted from now on are included.
Although in FIGS. 4 and 5 the number of antennas is one for the sake of simplicity, because two or more antennas are used for one channel when using a technology of MIMO, frames as shown in FIGS. 4 and 5 which corresponds to the two or more antennas, respectively, are multiplexed spatially.
An HT-SIG (HT SIGNAL field) is a field which only the radio communication terminal HT-STA and the radio communication terminal HT-MC-STA can recognize (i.e., a field which only the radio communication terminal L-STA cannot recognize), and includes information about the data which will be transmitted from now on.
Furthermore, an HT-SFT, an HT-LTF-1, and an HT-LTF-2 are training fields, and are used for, for example, estimating a transmission path for transmission and reception in communications using two or more antennas, such as MIMO communications.
A Data and an HT-DATA are fields in which actual data which are modulated according to the modulation method specified in the L-SIG or HT-SIG are included.
In the frame format of the radio communication terminal HT-MC-STA of FIG. 5, “Duplicate” shows that the same data as transmitted via a channel CH_a are transmitted via a channel CH_b.
Although the radio communication terminal L-STA can set up a time EIFS (Extended InterFrame Space) when it will complete its data transmission by calculating a time interval during which it will transmit data from information on the modulation method (Rate) and the data length (Length), which is included in the L-STF which is a common field, because regions and an overhead, such as HT-STF, HT-LTF-1, HT-LTF-2, and HT-DATA, which the radio communication terminal L-STA cannot recognize are included in each of the frame formats of the radio communication terminal HT-STA and the radio communication terminal HT-MC-STA, the radio communication terminal L-STA may calculate a wrong transmission time interval length so as to set up the EIFS even if it is going to calculate the actual data transmission time interval from the information about the modulation method (Rate) and the data length (Length) included in the L-SIG which is a common field so as to set up the EIFS.
To solve this problem, a method which is called “Spoofing” has been proposed, and, according to “Spoofing”, a combination of the modulation method (Rate) and the data length (Length) is derived and set to the L-SIG so that, in the formats of FIG. 4 and FIG. 5, the calculated data transmission time interval becomes equal to the actual data transmission time interval.
In this case, each of the radio communication terminal HT-STA and the radio communication terminal HT-MC-STA calculates the transmission time interval using HT-Rate and HT-Length for HT which are included in the HT-SIG or the like.
However, “Spoofing” is not used for control of either the sequence interval of transmission of two or more frames or 20 MHz operation and 40 MHz operation.
FIG. 6 is an explanatory diagram showing a “Channel Management” method in a prior art radio LAN system.
In FIG. 6, an example of using two channels is shown, and CH_a (Control) is a fundamental channel and CH_b (Extension) is an extended channel.
Assume that a radio communication terminal L-STA and a radio communication terminal HT-STA can run using only the channel CH_a (for example, a radio communication terminal L-STA and a radio communication terminal HT-STA in another BSS (i.e., a network which another access point creates) can run using only the channel CH_b), and a radio communication terminal HT-MC-STA can run using both the channel CH_a and the channel CH_b. For the sake of simplicity, the example in which the radio communication terminal L-STA and the radio communication terminal HT-STA can run using only the channel CH_a, and the radio communication terminal L-STA and the radio communication terminal HT-STA in the other BSS can run using only the channel CH_b will be shown. However, another example can be provided, and, for example, there can be various combinations of overlaps, such as a combination of a radio communication terminal L-STA and a radio communication terminal HT-STA, as shown in FIG. 10.
Because each of the channels CH_a and CH_b has a bandwidth of 20 MHz, each terminal performs a 20 MHz-bandwidth operation in a case of using the single channel CH_a or CH_b, whereas in a case of using both the channels CH_a and CH_b simultaneously, each terminal performs a 40 MHz-bandwidth operation.
For example, when a terminal control apparatus which is an access point prohibits data transmission by each of the radio communication terminal L-STA and the radio communication terminal HT-STA so as to enable the radio communication terminal HT-MC-STA to carry out data transmission with 40 MHz bandwidth, the terminal control apparatus carries out carrier sense CS of the channel CH_a, and, if the channel CH_a is being not used, generates radio data including transmission prohibit information for instructing prohibition of data transmission and transmits the radio data to both the radio communication terminal L-STA and the radio communication terminal HT-STA by using the channel CH_a.
FIG. 6 shows an example in which radio data are transmitted in the form of BCN (Beacon) or ICB (Increase Channel Band) frames. Because ICB frames are frames which are defined newly in order for a terminal control apparatus to carry out “Channel Management” with 20 MHz bandwidth and “Channel Management” with 40 MHz bandwidth, the existing radio communication terminal L-STA may not recognize ICB frames which are defined newly, and may not read the transmission prohibit information for instructing prohibition of data transmission.
If each of the radio communication terminal L-STA and the radio communication terminal HT-STA receives the radio data transmitted from the terminal control apparatus, and can read the transmission prohibit information included in the radio data, each of the radio communication terminal L-STA and the radio communication terminal HT-STA sets up virtual carrier sense information which is called NAV, and suspends data transmission which uses the channel CH_a during a predetermined time interval.
Next, when the channel CH_a and the channel CH_b are running independently, and the channel CH_b is being used (busy state) because of interference from another BSS or the like, the terminal control apparatus waits until the channel CH_b is released, carries out carrier sense CS of the channel CH_b, and, if it can check to see that the channel CH_b is being not used, transmits radio data including transmission prohibition information to both the radio communication terminal L-STA and the communication terminal HT-STA in the other BSS by using the channel CH_b.
FIG. 6 shows an example in which radio data are transmitted in the form of CTS (CTS-to-myself) or BCN (Beacon) frames.
When each of the radio communication terminal L-STA and the radio communication terminal HT-STA in the other BSS receives the radio data transmitted from the terminal control apparatus, and reads transmission prohibition information included in the radio data, it sets up NAV and suspends the data transmission using the channel CH_b during a predetermined time interval.
After the terminal control apparatus has prohibited the use of the channel CH_a or CH_b by each of the radio communication terminal L-STA and the radio communication terminal HT-STA in the above-mentioned way, it transmits radio data including prohibition release information for releasing the prohibition of the transmission to the radio communication terminal HT-MC-STA by using the channels CH_a and CH_b in order to release the prohibition of the data transmission by the radio communication terminal HT-MC-STA.
FIG. 6 shows an example in which radio data are transmitted in the form of CF-END frames.
When the radio communication terminal HT-MC-STA receives the radio data transmitted from the terminal control apparatus and then reads the prohibition release information included in the radio data, it releases the NAV, shifts to a state in which it can carry out communications, and then starts data transmission with 40 MHz bandwidth by using the channels CH_a and CH_b.
Next, in a case of prohibiting the data transmission by the radio communication terminal HT-MC-STA and then enabling each of the radio communication terminal L-STA and the radio communication terminal HT-STA to carry out data transmission with 20 MHz bandwidth, the terminal control apparatus transmits radio data containing transmission prohibition information to the radio communication terminal HT-MC-STA by using the channels CH_a and CH_b.
FIG. 6 shows an example in which radio data are transmitted in the form of DCB (Decrease Channel Band) frames. Because DCB frames are frames which are defined newly in order to carry out “Channel Management” with 20 MHz bandwidth and “Channel Management” with 40 MHz bandwidth, the existing radio communication terminal L-STA cannot recognize DCB frames which are defined newly. Furthermore, when frames are transmitted at a frequency band of 40 MHz, the radio communication terminal HT-STA cannot recognize the frames in the data region. However, the radio communication terminal HT-STA can recognize fields including up to the HT-SIG.
When the radio communication terminal HT-MC-STA receives the radio data transmitted from the terminal control apparatus and then reads the transmission prohibition information included in the radio data, it sets up NAV and suspends the data transmission using the channels CH_a and CH_b during a predetermined time interval.
After the terminal control apparatus has prohibited the use of the channels CH_a and CH_b by the radio communication terminal HT-MC-STA in the above-mentioned way, it transmits radio data including prohibition release information for releasing the prohibition of the transmission to both the radio communication terminal L-STA and the radio communication terminal HT-STA in the other BSS by using the channel CH_b in order to release the prohibition of the data transmission by each of the radio communication terminal L-STA and the radio communication terminal HT-STA in the other BSS.
FIG. 6 shows an example in which radio data are transmitted in the form of CF-END frames.
When each of the radio communication terminal L-STA and the radio communication terminal HT-STA in the other BSS receives the radio data transmitted from the terminal control apparatus, and then reads the prohibition release information included in the radio data, it releases the NAV, shifts to a state which it can carry out communications, and starts data transmission with 20 MHz bandwidth by using the channel CH_b.
The terminal control apparatus also transmits radio data including prohibition release information for releasing the prohibition of the transmission to both the radio communication terminal L-STA and the radio communication terminal HT-STA which the terminal control apparatus manages by using the channel CH_a in order to release the prohibition of the data transmission by each of the radio communication terminal L-STA and the radio communication terminal HT-STA.
FIG. 6 shows an example in which radio data are transmitted in the form of CF-END frames.
When each of the radio communication terminal L-STA and the radio communication terminal HT-STA receives the radio data transmitted from the terminal control apparatus and then reads the prohibition release information included in the radio data, it releases the NAV, shifts to a state which it can carry out communications, and starts data transmission with 20 MHz bandwidth by using the channel CH_a.    [Nonpatent reference 1] IEEE802.11 Standard, HyperLAN2 Standard    [Nonpatent reference 2] IEEE802.11e-Draft 13.0
A problem with the prior art radio LAN system which is so constructed as mentioned above is that when transmission prohibition information for instructing prohibition of data transmission is transmitted in the form of ICB frames which are defined newly in order for the terminal control apparatus to carry out “Channel Management” with 20 MHz bandwidth and “Channel Management” with 40 MHz bandwidth, the existing radio communication terminal L-STA cannot recognize ICB frames which are defined newly, and therefore cannot read the transmission prohibition information for instructing the prohibition of data transmission.
Another problem is that in order for the terminal control apparatus to carry out “Channel Management” with 20 MHz bandwidth and “Channel Management” with 40 MHz bandwidth, the terminal control apparatus must transmit a large volume of radio data to the radio communication terminal (in the example of FIG. 6, the terminal control apparatus transmits radio data six times), and therefore the overhead of channel switching becomes large.
A further problem is that the radio communication terminal HT-MC-STA can recognize the prohibition and release of 40 MHz-bandwidth data transmission if receiving radio data transmitted from the terminal control apparatus, whereas it cannot check to see whether or not the prohibition of 20 MHz-bandwidth data transmission is released, and therefore cannot switch between the 40 MHz-bandwidth data transmission and the 20 MHz-bandwidth data transmission.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a terminal control apparatus which can prevent an existing radio communication terminal L-STA from entering a state in which it cannot recognize control information and can also reduce the overhead of channel switching, and a radio LAN system.
It is another object of the present invention to provide a terminal control apparatus which can enable a radio communication terminal HT-MC-STA to carry out not only data transmission with 40 MHz bandwidth but data transmission with 20 MHz bandwidth, and a radio LAN system.